The present invention relates to air conditioning systems controlling temperature and humidity of air inside of residential, commercial, and industrial buildings and structures. In the heating mode, heat is delivered to a building either by an air conditioning system operating as a heat pump or by an alternative source of heat such as gas, electrical, solar, etc. heaters. In the cooling mode, the system absorbs heat from the “indoor air” and deliver this heat to ambient. Air conditioning systems in many buildings, especially, multi-story buildings, commercial buildings, hospitals, schools, supermarkets, etc. include supply-exhaust ventilation, a liquid circuit, and a refrigeration circuit. Liquid, mostly water or brine, used as a secondary refrigerant, circulates in the building providing fan-coil heat exchangers with heat from heated liquid in the heating mode or cold from cooled liquid in the cooling mode. In turn, fan-coil heat exchangers heat or cool air flowing through them and, therefore, warm or chill different parts of the building. In alternative design, a chiller may supply warmed or chilled liquid to other equipment, for example, local air conditioners or heat pumps wherein warmed liquid transfers heat to refrigerant in evaporators in the heating mode, or chilled liquid absorbs heat from refrigerant in condensers in the cooling mode.
Most of building air conditioners and chillers operate according to the Rankin refrigeration cycle. While majority of systems that condition air in buildings operate in both the heating and cooling modes, the present invention mostly relates to the cooling mode. In the cooling mode, the air is chilled and dehumidified while passing through cold surfaces of evaporators or fan-coil heat exchangers. In most air conditioners, the system running time is the only controlled parameter that is driven either by humidity setting or by temperature setting. Thus, humidity and temperature are never controlled independently of each other. While air conditioner runs, there is a relatively rigid correlation between percentage of cooling capacity dedicated to temperature reduction and percentage of cooling capacity for moisture condensation at any combination of indoor air humidity and temperature. The outdoor heat warms buildings primarily through the walls, roof, and windows while the moisture comes with outdoor air. Therefore, humidity in the room depends mostly on outside air influx when temperature setting is fixed. In a single-family residential building, air exchange rate because of leaks, cracks, open doors, etc. is between 0.5-2.0 volumes per hour. However, in the buildings with supply-exhaust ventilation air exchange rate is from minimum 4 to as high as 30. With so large infiltration of outside air to the building on a hot humid day, air influx may considerably increase indoor air relative humidity exceeding a recommended limit of 60% and, sometimes, even 70%. Inhabitants of such indoor environment feel discomfort; besides, high humidity may upsurge water accommodation in air ducts and mold on the walls. In order to reduce humidity, the indoor temperature is generally set below comfortable level to force the conditioning systems for longer run with higher percentage of cooling capacity spent on moisture condensation.
In this scenario, besides inhabitant comfort reduction, the air-conditioning system experiences increase in energy consumption and potential additional difficulties in draining considerable amount of condensed moisture from heat exchangers installed in the rooms.
Thus, development of a system with indoor air humidity control independent of temperature represents an important task for designers of the air-conditioning system, especially, the systems with supply-exhaust ventilation. Another aspect of air conditioner or chiller operations is heat utilization, for example, to heat or, at least, preheat water for technical or domestic consumption and/or for heat use in an air-conditioning system itself to reheat over-chilled air to comfortable level.
There are several methods for monitoring and reducing humidity of the supply airflow in order to control air humidity inside rooms. In small buildings with a single air conditioner, some designers position an evaporating coil in the supply air stream. There are advantages and disadvantages of this approach. However, creating several zones with independent control of conditioned air parameters is a necessity for large buildings with multiple rooms and different comfort requirements of inhabitants. To control humidity of supply air in large buildings, in addition to a main air conditioner or chiller, a custom air conditioner is installed in the building with its evaporator placed inside of the supply air duct specifically to cool and dehumidify incoming air. The problems of such system include extra installation and maintenance expenses for additional equipment. Besides, to elevate temperature of supply air to comfortable level, while the chiller is off or air after dehumidification is too cold, additional gas or electrical heating elements are also in use. That adds to operating expenses.
To reduce power consumption, some systems use heat from heated air leaving the conditioner either with an air-to-air heat exchanger or mixing supply air with a part of hot air leaving condenser. This saves energy compared to the systems with gas or electric heaters, but, in addition to aforesaid extra installation and maintenance expenses, such systems have several other disadvantages including:
a) positioning of the refrigerant evaporating coil inside the air duct often requires lengthy refrigerant communications, that, in turn, increases energy loss, reduces reliability, and complicates maintenance;
b) the air-to-air heat exchanger for reheating over-chilled air needs special ducts and a blower to harness a part of hot air after condenser and bring it either to the heat exchanger in air supply stream for heat transfer or for mixing with over-chilled supply air;c) high rise buildings with several technical floors for supply air ducts require either several conditioners or several evaporators with lengthy refrigerant lines and air-to-air heat exchangers or mixers.
Lately, air conditioners with a liquid desiccant circuit become popular for their high dehumidification capacity. U.S. Pat. No. 9,464,815 of Robert Uselton assigned to Lennox Industries offers an air conditioner with refrigerant and desiccant circuits. In the refrigerant circuit, refrigerant splits in two flows upstream from the evaporator. The first flow of refrigerant, enters an absorber through the first evaporating coil, and evaporates in the absorber cooling liquid desiccant that absorbs water vapor from air. The liquid desiccant is located inside the absorber case with a frame on the perimeter and membranes permeable for vapor and resistant for liquid on the sides. The second flow of refrigerant evaporates in an evaporator cooling air that circulates in the room. Air cooled in the evaporator reaches the absorber that absorbs water vapor, and then dry cooled air flows back to the room. A liquid pump moves desiccant with absorbed moisture to a desorber. Vapor refrigerant evaporated in the first evaporating coil and vapor refrigerant from the evaporator merge in a single line that goes to the compressor suction. Hot refrigerant vapor compressed in the compressor flows to a discharge line where it splits into two branches: the first branch delivers refrigerant to a desorber condensing coil located in the desorber and downstream of the desorber merges with the second branch, then refrigerant from both branches flows to the condenser. In the desorber, moisture absorbed by liquid desiccant in the absorber is evaporated from desiccant while desiccant is warmed up with heat received from the desorber condensing coil. In the condenser, refrigerant is condensed rejecting heat to ambient air flowing through the condenser. Air heated in the condenser flows around the desorber while picking up water vapor penetrated through the desorber membranes and carrying water vapor to ambient.
The design described in the U.S. Pat. No. 9,464,815 is free from many problems caused by use of an extra air conditioner, specifically, for moisture removal. However, this design has several disadvantages also. First, it is complicated and expensive. Second, membranes could have oleophilicity that leads to severe fouling of the membranes surface. Third, the membranes require maintenance, at least, dust cleaning. Forth, while the design is applicable to small buildings with limited area and number of rooms, it is not suited for large multi-story buildings because these buildings often require multiple supply and exhaust air ducts, and, therefore, delivering refrigerant coils for absorbing and desorbing moisture into/from desiccant is extremely complicated. There is also a problem with air humidity control. In spring or fall, or even on dry summer days, amount of moisture removal shall be reduced. One solution is to reduce desiccant pump speed; however, in this case, heat absorption from the desiccant condensing coil also drops that leads to reduction in amount of condensed liquid refrigerant. While there is also reduction of liquid refrigerant in the evaporator, still reduction in condensed refrigerant can trigger imbalance in amount of liquid refrigerant that, in turn, leads to loss of the cooling capacity and/or the operating efficiency decrease.
Other disadvantages of existing air-conditioning systems are relevant to the methods used for refrigerant condenser heat utilization. These methods include the following:                Use of heat delivered to users with air heated in an air-cooled condenser. As mentioned above, this method leads to additional initial and operational expenses because it requires a bulky air-to-air heat exchanger with an extra blower and special air ducts.        Use of a water-cooled heat exchanger as the condenser delivering heat from heated water to users. It is a very efficient way of utilization when users exist for all condenser heat or its considerable part. However, in most buildings, heat requirements hardly exceed 5-10% of heat available from condensing. It means that an additional water-to-air heat exchanger or a cooling tower for cooling water circulating through the condenser must be installed. Besides of additional installation and operational expenses, it may lead to condensing temperature and compressor power increase.        Installation of a dedicated water-cooled heat exchanger located either downstream of the compressor and upstream of the condenser or downstream of the condenser. Heat exchanger upstream of the condenser operates as an additional part of the condenser desuperheating refrigerant vapor and sometimes condensing a part of this vapor. It requires additional refrigerant charge. However, while there is no need in utilized heat, extra refrigerant in the condenser causes discharge compressor pressure and power increase. If additional heat exchanger is downstream of the condenser, it operates as a subcooler cooling liquid refrigerant after the condenser and utilizing heat carried by water. This method has many advantages when compared to methods discussed above. With relatively small expenses, users can get utilized heat for air temperature control and for other purposes. Besides, this design brings improvement in the refrigeration circuit efficiency. For example, only 5% of condensing load absorbed in the subcooler may increase capacity and efficiency by 10%, while the same amount of heat utilized in the heat exchanger upstream of the condenser brings improvement of less than 1%. However, use of the subcooler is restricted by level of heat utilization. Requirement in utilized heat normally varies and can drop as low as zero; at these conditions, the subcooler becomes just a container filled with liquid refrigerant. The system refrigerant charge is optimized for subcooling. Thus, reduction in evaporator capacity and because of this, reduction of liquid refrigerant in evaporator increases amount of liquid refrigerant in the condenser, in turn, that leads to condensing pressure and compressor power increase.        
The present invention eliminates aforementioned disadvantages with a chiller that is energy efficient, inexpensive, reliable, universal, and convenient for maintenance. The presented system is capable of deep dehumidification of the supply air with comprehensive control of this air temperature, and very efficient utilization of heat generated by the chiller condenser.